The present invention relates to a drive for driving a load in an electrical apparatus and to electrical apparatus incorporating such a drive. It can be used in laundry appliances such as washing machines or washer-dryers, but is not limited to use in such appliances.
Motors are widely used in appliances for a variety of purposes. In laundry appliances such as washing machines, washer-dryers and dryers, a motor is used to rotate a drum which holds clothes. The drum is required to rotate at a range of speeds, from a relatively slow rotational speed of around 50 rpm during a washing cycle to a fast rotational speed of 1600 rpm or more during a spin cycle. The washing load within the drum can be 11 kg or more in a domestic machine. Clearly, the motor which rotates the drum is required to operate over a wide range of conditions. The motor is typically an ac motor which is powered by the mains power supply. In Europe, the ac motor is usually a universal motor.
It is known, in the field of traction motors for vehicles, to drive two dc motors in a way that switches between driving the motors in series or in parallel according to the speed and torque requirements of the vehicle at a particular time. In both the series and parallel arrangements, the motors are driven with a dc supply. UK Patent 1,422,161 shows an example of this kind of arrangement.
The present invention seeks to provide an improved way of operating an electrical apparatus that includes two motors.
Accordingly, a first aspect of the invention provides a drive for driving a load in an electrical apparatus, the drive comprising first and second motors for coupling to a power supply, outputs of the motors being used to drive the load, and switching means for switching the motors between:
a series configuration, in which the motors are connected in series across the power supply and driven with a dc supply; and,
a parallel configuration in which the motors are connected in parallel across the power supply and driven with an ac supply.
This arrangement has the advantages that the current drawn at low speed is half that which would be required with the motors connected in parallel, with an associated reduction in absolute current harmonics. Furthermore, at low speed, the overall terminal voltage of the series-connected motors is double that of parallel-connected motors, giving a smaller firing angle (i.e. greater conduction period) which reduces normalised harmonic content and improves the power factor. Wear of the two motor brushes is unequal with dc, reducing the life of the motor significantly. Switching to ac at high speed, at which most brush wear occurs, gives more even brush wear and so a longer motor life.
Preferably the power supply is an ac supply and, in the series configuration, the switching means switches a rectifier across the ac supply to provide the dc supply. Freewheeling current through the bridge rectifier during the time that the power switching device (triac) is switched off gives a higher r.m.s. current on the dc side of the bridge rectifier than on the ac side, and therefore greater motor torque than would normally be expected from a given ac input current This has the advantage that smaller-stack motors can be specified than would otherwise be required to give the same output torque with ac supply at low speed. Also, the bridge rectifier and associated components only need to be rated to take the current when the motors are connected in series, thus reducing the component cost and size. Furthermore, as the bridge rectifier dissipates heat only during low-speed operation, a smaller heat sink is required. Switching the bridge rectifier out of circuit at higher speeds means that under these conditions an inductive load is seen by the supply and the firing angle is reduced, which together reduce the harmonic content of the supply current.
The described embodiments show a laundry appliance with the motors being used to rotate a drum which holds clothing items, but the invention can be applied to other forms of electrical apparatus.
Another aspect of the invention provides a method of operating a drive for driving a load in an electrical apparatus.